This application claims the priority of Korean Patent Application No. 2002-65659, filed on Oct. 26, 2002 in the Korean Intellectual Property Office, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a membrane-electrode assembly (MEA) of a fuel cell, and more particularly, to an MEA of a fuel cell having an enhanced interface characteristic between a polymer electrolyte and electrodes, a production method of the same, and a fuel cell employing the same.
2. Description of the Related Art
Recently, as portable electronic devices and wireless communication devices have developed and vehicles using fuel cells are being developed, the development of high performance fuel cells that are reliable and operate at a low temperature, for example, an ambient temperature, are in demand.
Among fuel cells, there are fuel cells which use polymer electrolyte. PEMFC (proton exchange membrane fuel cell) use hydrogen as anodic fuel and DMFC (Direct methanol fuel cell) use liquid methanol fuel. Liquid methanol fuel is supplied directly to anodic fuel. In such polymer electrolyte fuel cell, a polymer electrolyte membrane is assembled between a cathode and an anode. The polymer electrolyte membrane has to secure low resistance, excellent mechanical property and chemical stability.
The unit of the fuel cell, i.e., membrane-electrode assembly (MEA), is formed by an electrolyte layer and electrodes that are arranged at both sides of the electrolyte layer to generate an oxidation reaction and a reduction reaction. In addition, each of the electrodes includes a support and a catalyst layer. The catalyst layer reacts with the fuel for electrochemical reaction and produce current. The support layer distributes the fuel and maintains the shape of catalysts layer and strength of the unit cell.
Generally, two methods are known to produce an MEA of a fuel cell.
First, after forming electrodes by placing catalyst layers on electrode supports, the electrodes and a polymer electrolyte membrane are hot pressed at a high temperature and under a high pressure so that interfaces between the polymer electrolyte membrane and the electrodes are formed as disclosed in U.S. Pat. No. 6,180,276.
However, where a Nafion membrane is used as a polymer electrolyte membrane and is directly used in a methanol fuel cell or a polymer electrolyte fuel cell, water retention is limited at a high temperature and an ionic conductivity is reduced so that the efficiency of the fuel cell is lowered.
Second, after directly coating and drying a catalyst layer on a Nafion membrane, electrode supports are directly hot pressed to form an MEA, as disclosed by John Appleby and S. Gamburzev in Journal of Power Source, Vol. 106 Issues 1–2, pp. 206–214.
The conditions of temperature and pressure in the hot pressing are varied depending on the kind of the polymer that is included in the polymer electrolyte membrane. When a Nafion membrane is used as the polymer electrolyte membrane, an interface between the polymer electrolyte membrane and the electrodes is favorably formed. However, when other material, particularly, a rigid polymer, is used as the polymer electrolyte membrane, the performance of the fuel cell may be deteriorated.
For example, where a sulfonated polymer is used, it is difficult to hot press the sulfonated polymer electrolyte membrane because the sulfonated polymer is brittle. In addition, in the case where a proton conducting polymer is used, there is a difference in the physical property depending on water retention characteristic. For example, the proton conducting polymer cannot retain water at a high temperature, which causes a change in the physical property. Accordingly, when such a polymer is hot pressed at a high temperature and under a high pressure, the physical property of the polymer deteriorates and undesirable problems occur.